In the past, BH steel sheets of a TS: 340 MPa class bake-hardenable steel sheets (hereinafter referred to as “340BH”) have been applied to automobile exposure panels, such as hoods, doors, trunk lids, backdoors, and fenders, which are required to have excellent dent resistance. The 340BH is ferrite single phase steel in which in ultra low carbon steel containing less than 0.01% of carbon, the amount of solute carbon is controlled by addition of carbide or nitride forming elements such as Nb and Ti, and solid solution strengthening is performed by addition of Si, Mn, and P. In recent years, from the further demand for the weight reduction of automotive bodies, various investigations have been carried out to increase the strength of the exposure panels to which the 340BH has been applied, to achieve a reduction in the thickness of the steel sheet, a weight reduction in the R/F (Reinforcement: inner reinforcement parts) with the same thickness of outer panels, a reduction in the temperature and the time in a bake coating process, and the like.
However, if large amounts of Si, Mn, and P are further added to the conventional 340BH to increase the strength thereof, surface distortion of pressed parts considerably deteriorates due to an increase in YP. In this case, the surface distortion indicates minute wrinkles and/or undulated patterns which are likely to occur on a press formed surface such as a periphery of a door knob portion. Since the surface distortion remarkably degrades the appearance quality of automobiles, steel sheets applied to the exposure panels are required to have low yield stress before press forming which is close to YP of the present 340BH while the strength of a press formed product is increased.
In addition, in press forming of the parts, although bending is performed on a flange portion to join to inner parts, if the ductility of an edge of a sheared or punched blank, that is so-called “stretch-flangeability,” is not sufficient, cracks generate at the edge. For example, if the stretch-frangeability is deteriorated by increasing tensile strength from that of 340BH, cracks often generate on the edge of flanges by hemming the flanges of periphery of backdoors or window frame of doors, or by bending the edge of flanges of fenders to join to side-panels. Accordingly, a steel sheet used for the applications as described above is required to have excellent stretch flangeability.
Furthermore, a steel sheet used for automobiles is also required to have excellent corrosion resistance. Since steel sheets are closely in contact with each other at a hem processing portion and a spot welding peripheral portion of body parts such as a door, a hood, and trunk lid, chemical films are difficult to form by electrocoating. Hence, rust easily forms. In particular, in corner portions at a front side of a hood and a lower side of a door, at which water is liable to remain and which are exposed to a wet atmosphere for a long time, holes are frequently generated by rust. Furthermore, in recent years, car body manufactures have been considering increasing the hole-forming resistant life to 12 years from a conventional life of 10 years by improving corrosion resistance of car bodies. Hence, a steel sheet must have sufficient corrosion resistance.
Because of these circumstances, for example, in Japanese Examined Patent Application Publication No. 57-57945, a method for obtaining a high-strength steel sheet of a 340 to 490 MPa grade is disclosed in which the amount of Ti is controlled in steel containing 0.02% or less of C so that Ti(%)/C(%)≧4.0 holds, and large amounts of Si, Mn, and P are added.
In addition, Japanese Examined Patent Application Publication No. 62-40405 discloses a method for obtaining a galvannealed steel sheet having both a low yield stress (YP) and a high ductility (El) by appropriately controlling a cooling rate of steel containing 0.005% to 0.15% of C, 0.3% to 2.0% of Mn, and 0.023% to 0.8% of Cr after annealing to form a dual phase microstructure primarily formed from ferrite and martensite.
In addition, Japanese Patent No. 3613129 discloses that when the total amount of Mn, Cr, and Mo is set to 1.8% to 2.5% in steel containing 0.02% to 0.033% of C, 1.5% to 2.5% of Mn, 0.03% to 0.5% Cr, and 0% to 0.5% Mo, a steel sheet having a YP of 300 MPa or less, excellent ductility (El), and excellent stretch flangeability (hole expanding ratio, λ) is obtained.
Japanese Unexamined Patent Application Publication No. 8-134591 discloses a method for obtaining a high-strength galvanized steel sheet having a tensile strength of a 440 to 590 MPa class and excellent stretch flangeability (hole expanding ratio, λ) in which the total amount of Mn and Cr of steel containing 0.02% to 0.14% of C, 1.3% to 3.0% of Mn, and 0.3% to 1.5% of Cr is set to 2.0% to 3.5%, and a microstructure of the steel sheet is formed as a multi phase, on an area ratio basis, of 50% or more of a ferrite phase, 3% to 15% of bainite, and 5% to 20% of martensite.
Japanese Unexamined Patent Application Publication No. 2008-19502 discloses a method for obtaining a steel sheet having a low yield ratio, high BH, and excellent room-temperature anti-aging property which is obtained by setting Cr/Al to 30 or more in steel containing 0.02% to 0.08% of C, 1.0% to 2.5% of Mn, 0.05% or less of P, and more than 0.2% to 1.5% of Cr.
In Japanese Unexamined Patent Application Publication No. 2006-233294, a method for obtaining a steel sheet having a low YR and high bake-hardenability is disclosed in which steel containing 0.01% to less than 0.040% of C, 0.3% to 1.6% of Mn, 0.5% or less of Cr, and 0.5% or less of Mo is cooled to a temperature of 550° C. to 750° C. at a cooling rate of 3° C. to 20° C./s after annealing and is cooled at a cooling rate of 100° C./s or more to a temperature of 200° C. or less.
However, since the steel sheet disclosed in JP '945 is IF steel in which C is stabilized by Ti and is ferrite single phase steel, as a strengthening mechanism, solid solution strengthening of Si, Mn, and P must be inevitably used Hence, YP is increased by adding large amounts of these elements, and appearance quality and powdering resistance of zinc-coated steel sheets are remarkably degraded.
The methods described in JP '405 and JP '129 disclose the steel in which an appropriate amount of a second phase primarily composed of martensite is dispersed in a ferrite microstructure, and YP is decreased as compared to that of solid solution strengthened steel, such as conventional IF steel. However, when press forming is performed on these steel to form body parts such as a door, there are many steel sheets having a large surface distortion amount as compared to that of the conventional 340BH. Hence, a further decrease in YP is required. In addition, since steel sheets often accompany cracks after bending of a flange end, further improvement in stretch flangeability is also required. Furthermore, when we investigated corrosion resistance of actual parts such as hoods and doors using the steel described above, it became clear that some steel sheets described in the examples had remarkably inferior corrosion resistance to that of the conventional 340BH at a portion at which the steel sheets were in close contact with each other. In addition, large amounts of expensive elements such as Cr and Mo, are added to many steel sheets described in these examples. Hence, the costs thereof remarkably increased.
In addition, since the steel described in JP '591 includes bainite as a microstructure, YP is high, and sufficient surface precision of pressed parts cannot be obtained. Furthermore, as in the case described above, it became clear that many steel sheets described in the examples had insufficient corrosion resistance.
Since Cr is positively used, the steel described in JP '502 has a relatively low YP and a high hole expanding property. However, as in the case described above, it became clear that many steel sheets described in the examples had insufficient corrosion resistance. In addition, since large amounts of expensive elements such as Cr and Mo, are added to these steel sheets, the costs thereof increased.
In addition, since the method described in JP '294 requires rapid cooling after annealing, it can be applied to a continuous annealing line (CAL) which performs no plating treatment. However, it is theoretically difficult to apply the above method to a current continuous galvanizing line (CGL) in which a plating treatment is performed by immersing a steel sheet in a galvanizing bath held at 450° C. to 500° C. during cooling after annealing.
As described above, a galvanized steel sheet which can satisfy all requirements, good corrosion resistance, a low YP, and excellent stretch flangeability, could not be obtained by conventional techniques.
It could therefore be helpful to provide a high-strength galvanized steel sheet which does not require addition of large amounts of expensive elements such as Mo and Cr, and which has excellent corrosion resistance, a low YP, and good stretch flangeability, and a method for manufacturing the same.